for me, i think very scientifically, evolution, in that case it must be the egg, because a chicken evolved from another bird slightly different than a chicken. sorry, don't want to offend anyone religious, i respect your beliefs :]

egg. like a couple people have mentioned, a slightly less-evolved life form or whatever would have laid the egg the first chicken came from. In reverse, i dont really see how the chicken could have come without coming from an egg first.